An example of an electrically-controlled unit fuel injector is shown in U.S. Pat. No. 4,392,612 issued to Deckard, et al. on Jul. 12, 1983. In Deckard, et al. the injector includes a mechanically-actuated fuel pumping plunger and an electrically-actuated fuel pressure control valve assembly. The pressure control valve assembly includes a solenoid-operated poppet valve that controls fuel pressure in the unit injector in order to control fuel injection delivery. Fuel pressure is controllably enabled to be developed within the injector by electrical actuation of the pressure control valve valve assembly. Fuel pressure is controllably prevented from developing within the injector by not electrically actuating the pressure control valve assembly.
In such electronically-controlled unit injectors, the solenoid is electrically energized in response to an electronic control module and the armature of the pressure control valve assembly moves the poppet valve in one direction until it engages a valve seat. The solenoid is maintained electrically energized and holds the poppet valve in the fuel sealing position to enable fuel pressure to be developed in the unit injector, eventually resulting in fuel injection when a predetermined valve opening pressure (VDP) is reached. At the end of the fuel injection cycle, the solenoid is electrically deenergized and a return spring backs the poppet valve off of the valve seat and returns the poppet valve to the valve open position which prevents the development of fuel pressure by spilling the fuel back to the fuel reservoir.
Several problems have been noted in these presently available pressure control valve assemblies and it is desired to seek solutions thereto. First of all, the electronic control module or associated driver(s) must supply sufficient power to the solenoid coil to maintain the poppet valve in the fuel sealing position so that fuel injection can be achieved. It is desired to minimize the holding power requirements not only because this would lessen the energy required, but also to achieve faster valve opening.
Secondly, presently available pressure control valve assemblies are slow to close because the electromagnetic force produced by the electrical solenoid coil must overcome the return spring force to close the poppet valve. The return spring force is relatively constant during the stroke of the poppet valve because the stroke is relatively small and therefore continually opposes the electromagnetic coil force acting to move the poppet valve toward closure.
Also, presently available pressure control valve assemblies require a relatively large number of piece parts so that they are expensive to manufacture and slow in operation. Furthermore, the relatively large number of piece parts in presently available units tends to increase manufacturing costs and reduce reliability.
Because currently available fluid control valves are relatively slow, they can not give fast enough responses to meter very small quantities of fuel per injector stroke. Split fuel injection, which is desired because it improves engine thermal efficiency, reduces engine noise emissions, and lowers the emission of undesired oxides of nitrogen (NO.sub.x), requires metering very small quantities of fuel. Therefore current fluid control valves can not provide the desired split fuel injection.
The present invention is directed to overcoming one or more of the problems as set forth above.